The present invention relates to a slab geometry laser device (otherwise known as a total internal reflection, face pumped laser device), and more particularly to a specific structure for holding the slab geometry crystal material.
The total internal reflection, slab geometry laser is based on concepts that result in beam quality improvements over conventional rod geometry solid state lasers. All solid state lasers suffer from thermally induced beam defocusing, birefringence and depolarization due to flash lamp pumping. The key difference between the two crystal geometries is that in the rod geometry crystal, the thermal gradient is radial, whereas in the slab geometry crystal, the thermal gradient is one dimensional.
In slab geometry lasers, the crystal cross section is rectangular with an aspect ratio of greater than 2:1 (long side to shorter side). The faces with the largest area are exposed to the flashlamp, thus setting up a gradient in the direction along the shorter side line; the direction of the gradient being away from the centerline of the crystal cross section. If the generated laser wavefronts were to pass straight through the slab crystal as it does through the rod, there would obviously be no benefit with the rectangular cross section. The benefit comes when the light is introduced at an angle into the crystal so that the wavefront is reflected internally between the crystal faces. It is crucial that the wavefront passes through the centerline of the crystal an even number of times so that the wavefront experiences a refractive gradient which is opposite in direction to the gradient it encountered before the internal reflection, therefore, any effects due to birefringence are effectively canceled. Of course, this is assuming that there is a perfectly uniform thermal gradient in said direction. In reality, the sides of the crystal that are not being pumped are not completely isolated thermally, and so there may be a small gradient in the direction (along the longer side line). The effects will be most noticeable at the corners of the crystal, as seen from the output.
As was suggested in the previous discussion, a one dimensional thermal gradient can only be achieved if the sides of the slab are insulated against heat transfer from the side of the crystal. In addition to this, the sides of the crystal must be sealed from the cooling water that makes contact with the crystal faces. The obvious choice of material which satisfies both requirements is an elastomer capable of withstanding relatively high temperatures (up to 100.degree. C.), and exposure to ultraviolet radation.
The motivation for the particular design of the slab holder in this invention is that it is modular in that it can be removed from the pump station without disturbing any other pump station parts. The motivation for this invention is that the crystal need not be permanently fixed to the holding device. Before attempting to achieve the modularity, specific system requirements must serve as guidelines.
The first guideline is that no water can make contact with any of faces to be used in the path of the beam. This is an obvious necessity, but one that is difficult to achieve. Previously, the method used to mount the crystal into the crystal holder consisted of using a sealant such as a silicone elastomer to provide a stress free seal between the crystal and the crystal holder. Two problems arise when this method is used. The first problem is that the mount is semipermanent, and should the crystal need to be removed from the mount; care must be taken to remove the sealant. The second problem arises from the fact that silicone sealants contain acids which could damage certain crystals during the curing phase of the sealant. This invention avoids these problems through the use of a translucent elastomeric o-ring coupled with some unique elements to from a non-permanent seal.
The second guideline specifies that the sides of the crystal must be sealed and insulated. This means that the entire length of the crystal will have insulation on the sides, and implies that the o-rings at the end of the crystal will pass over the insulating material at some point. This is not an ideal situation; at the same time, it is not prohibitive as long as the insulating material, the crystal, and the crystal retaining rails form a flush surface which the o-ring will pass over.
Ideally, the o-ring should only make contact with one continuous surface. In this invention, the o-ring makes contact with three different surfaces, and it is not obvious that this will form a successful seal. The prototype demonstrates, however, that as long as the insulation layer is thin, somewhat harder that the o-ring, and forms an even surface between the crystal and the side rail, the seal will be successful.
The third guideline requires that a maximum amount of area is to be exposed to the flashlamps. This leaves the holder with few contact areas with which to contain and seal the crystal. In this particular design, the crystal side retaining rails will be fabricated so that there is a slight overhang so that it may position the crystal, and protect the insulation material from being directly exposed to flashlamp radiation.
Accordingly, an object of the present invention is to make a holding device that is compact and easy to remove from, or place into, the pump station.